Spatial frequency is a measure of how rapidly a parameter changes over distance in a prescribed spatial direction and is analogous to temporal frequency, which is a measure of how rapidly a parameter changes with the passage of time. In television systems using horizontal scanning lines, horizontal space is conformally mapped to time by the scanning process, so horizontal spatial frequency of the televised image intensity conformally maps to temporal frequency in the video signal descriptive of the televised image.
In video cameras using a single pickup device, a color pattern filter may be used to filter the light reaching the pickup device so that color signals can be extracted from the electrical signal supplied by the pickup device. The color pattern filter customarily contains stripes transmitting light of three different colors to the pickup device, which may be a vidicon or may be a solid-state imager such as line-transfer charge-coupled device. The direction of the stripes is perpendicular to the direction of line scan in the camera, line scan conventionally being in a horizontal direction. The stripes of each color are of uniform width, but the stripes of different colors are preferably of different widths to simplify the separation of color components from the output signal of the pickup device. The respective widths associated with the different colors are usually scaled in regard to the contribution of the particular color to luminance--that is, to reference white. If the color filter comprises red-transmissive, green-transmissive and blue-transmissive stripes, for example, the green-transmissive stripes will be the widest and the blue-transmissive stripes will be the narrowest. The signals picked up by the narrower width stripes have poorer signal-to-noise ratio (S/N), particularly in the higher horizontal spatial frequencies containing detail. When the video camera is used with a video transmission system where the color signals are converted to wideband luminance and narrowband color-difference signals, the poorer S/N of the colors contributing less to luminance is not of much concern, since detail enhancement or video peaking is usually carried out on the shared luminance high frequencies rather than on individual color signals.
However, the video camera can be used with video equipment in which the color signals are not combined to form luminance and color-difference signals--e.g. certain digital video transmission systems of the so-called RGB type where the red (R), green (G) and blue (B) color signals are separately digitized and coded. In such equipment detail enhancement or video peaking is apt to be done on the red (R), green (G) and blue (B) color signals themselves. Since the human visual system discriminates poorly between colors of detail as details become finer, the enhancement of the color details that have poorer S/N with the color details that have better S/N can result in images that have less apparent noise in them. Random noise in the green (G) color signal is not correlated with random noise in the red (R) and blue (B) color signals, so on average the random noise component of the G signal and the random noise component of another color signal add as quadrature vectors rather than in-phase vectors, which apparently helps the high-frequency S/N when enhancing the detail of that other color signal.
FIG. 1 shows detail enhancement circuitry used in the prior art, in which an image signal input to the green (G) channel is delayed by cascaded first and second delay lines 1 and 2, each providing a delay equal to the duration (1H) of a horizontal scan line. Thereafter, the original (undelayed) image signal, a first output image signal delayed by 1 H period by the first 1 H delay line 1, and a second output image signal delayed 2 H periods by means of the first and second 1 H delay lines 1 and 2 are applied to a vertical highpass filter (V HPF) weight-and-sum circuit 3 for extracting a vertical detail component. Meanwhile, the 1 H-delayed image signal delayed by the first 1 H delay line is also applied to a horizontal highpass filter (H HPF) 4, thereby extracting a horizontal detail component.
The vertical detail component supplied from the V HPF 3 is filtered by a horizontal lowpass filter (H LPF) 5 to prevent diagonal detail from being excessively enhanced, as will be explained in more detail further on in this specification. Then, the detail components respectively extracted by H HPF 4 and by H LPF 5 are summed in an adder 6, to be applied as addressing to a read-only memory (ROM) 7 storing a look-up table (LUT).
The LUT stored in ROM 7 executes a process having a system characteristic which is related to the detail component level of the image signal. This process may take several forms, ranging from a simple linear amplification of the detail component to a non-linear attenuation of the noise component included in the detail component. Such a process can also include coring of the detail component at a level somewhat above its thermal noise level, thus providing a video detail signal in which thermal noise is suppressed to the enhanced-detail signal supplying means composed of second, third and fourth adders 8, 9 and 10. Also, the LUT stored in ROM 7 can execute a non-linear coring process. That is to say, input levels lower than the thermal noise level are set to zero while those higher than the thermal noise level are amplified to enhance the detail, and those much higher than the thermal noise level are attenuated to prevent over-enhancement of the detail.
The detail enhancement component supplied by the ROM 7 is added to each of the image signals of the red (R) and blue (B) channels, and to the output signal of the first delay line 1, in the second, third and fourth adders 8, 9 and 10, respectively. The summed image signals are then supplied as red, green and blue enhanced-detail signals R', G' and B' for the respective channels.
The detail enhancement circuit shown in FIG. 1 is designed such that, using H LPF 5, a diagonal detail component is not overly enhanced. That is to say, when bright pixels are distributed horizontally in the P1, P2 and P3 pixel positions in the 1 H line, as shown in FIG. 2A, the vertical detail component associated with these pixels is detected by V HPF 3; and there is no horizontal detail component associated with these pixels to be detected by H HPF 4. When bright pixels are distributed vertically in the P2 pixel position of the 0 H, 1 H and 2 H lines, as shown in FIG. 2B, the horizontal detail component associated with these pixels is detected by H HPF 4 and there is no vertical detail component associated with these pixels to be detected by V HPF 3. However, when bright pixels are distributed diagonally, as shown in FIG. 2C, the vertical detail component associated with these pixels is detected by V HPF 3 and the horizontal detail component detail component associated with these pixels is detected by H HPF 4. So diagonal detail enhancement is excessive if done in response to the summed responses of V HPF 3 and H HPF 4. Accordingly, a horizontal lowpass filter (H LPF) 5 is connected in cascade after V HPF 3 to perform a lowpass filtering operation that suppresses the vertical detail component when it is associated with a horizontal detail component and thereby curbs the undesirable tendency towards excessive detail enhancement. Isolated pixels differing from their neighbors are still enhanced excessively, however.
Delay (not shown in FIG. 1 ) must be included in the G input connection to the fourth adder 10 from the output connection of the 1 H delay line 1, in order to compensate for delay through H HPF 4; this delay can be provided by tapping the delay line 2, or replacing it by cascaded delay lines that in effect provide a tapped delay line 2. Respective delay lines (not shown in FIG. 1 ) are customarily included in the R input connection to the second adder 8 and in the B input connection to the third adder 9 to provide delays of the R and B signals compensating for the delay of the G signal as supplied to the fourth adder 10. The delay through H LPF 5 can compensate for the delay through H HPF 4.
In an alternative design for curbing the undesirable tendency towards excessive detail enhancement, which does not use H LPF 5, but instead supplies input signal to H HPF 4 from a vertical lowpass filter receiving input signals similar to V HPF 3, additional compensating delay has to be introduced after V HPF 3 and in each of the R, G and B inputs to the adders 8, 9 and 10. The FIG. 1 prior-art design for curbing the undesirable tendency towards excessive detail enhancement is the preferred one of these two equivalents because of its reduced requirement for compensating delay, particularly after V HPF 3.
There are problems with the FIG. 1 detail enhancement circuit having to do with detail enhancement, or peaking, reducing the signal-to-noise ratio (S/N) of images reproduced from the red, green and blue enhanced-detail signals R', G' and B'. This is the reason that the measurement of the S/N ratio of devices including a detail enhancement circuit (e.g., a camera) is customarily done so that detail enhancement is not done at the same time. The human visual system tends to ignore reduced S/N in the presence of detail, but is quite sensitive to high-spatial-frequency noise occurring in areas of an image where there is relatively little detail. Arranging for the LUT stored in the ROM 7 to supply a cored video detail signal avoids much of the reduction of S/N otherwise caused by enhancing detail. However, it still may be desirable to core the R', G' and B' signals to reduce their high-spatial-frequency noise content under conditions where there is not appreciable detail enhancement, when these signals are generated from R, G and B signals originating from a video camera receiving little light, for example.
There are also problems with the FIG. 1 detail enhancement circuit having to do with enhancement of the horizontal and vertical detail components in each of the red (R), green (G) and blue (B) channels depending solely on the image signal of the green (G) channel. The detail enhancement suitable for the characteristics of the respective channels is not achieved. By way of example, detailed red and blue color patterns cannot be enhanced, since there are no green details with which to provide enhancement.
The inventor has found that enhancement of the red (R) channel details can be carried out satisfactorily in response to the details originally appearing in the red (R) channel, without unacceptably lowering high-frequency signal-to-noise ratio in the red (R) channel, if noise coring of the red (R) channel details is done properly. The inventor has also found that enhancement of the blue (B) channel details can be carried out in response to the details originally appearing in the blue (B) channel, without unacceptably lowering high-frequency S/N in the blue (B) channel, if noise coring of the blue (B) channel details is done properly. Noise coring of the green (G) channel details also improves high-frequency S/N in that channel. Accordingly, it is possible to enhance the red (R) channel details in response to the details originally appearing in the red (R) channel and to enhance of the blue (B) channel details in response to the details originally appearing in the blue (B) channel, as well as to enhance the green (G) channel details in response to the details originally appearing in the green (G) channel, which procedure avoids problems with the prior art detail enhancement circuit caused by details that are not related primarily to luminance variation.
The red (R) channel high frequencies, separated for use in enhancing detail, can also be used for suppressing high-frequency noise in the red (R) channel when there is very little or no red detail. The green (G) channel high frequencies, separated for use in enhancing detail, can also be used for suppressing high-frequency noise in the green (G) channel when there is very little or no green detail. And the blue (B) channel high frequencies, separated for use in enhancing detail, can also be used for suppressing high-frequency noise in the blue (B) channel when there is very little or no blue detail. This noise-suppression scheme for the fullband R, G and B color signals is useful even when they are subsequently matrixed into a composite video signal using mixed highs.